A typical optical network, for example a dense wavelength division multiplex (DWDM) network, has many passive and active optical components. These optical components should have stable characteristics for reliable network operation. In particular, the optical components should have stable position and width of optical frequency passbands. Various factors may alter the frequency passbands of the optical components and can cause, for example, a frequency shift/offset of the passband resulting in an asymmetric narrowing of the passband. When propagating through many such components, a wavelength channel signal can undergo a significant spectral narrowing, causing penalties and faults in the optical network.
Improvements are therefore needed in monitoring performance of optical network equipment and, in particular, spectral performance of optical networks passive and active optical filtering and wavelength selective network components and modules, to improve spectrum utilization efficiency of optical networks.